Ooooops, I found the culprit and its ME.!
The instruction manual says to insert 3 balls, but this obviously is not correct.
I took one ball out and the game runs perfectly.

 When the game froze, I found that if I switched off the "right ball ramp switch" a new ball would be ejected.
This got me thinking this switch needed to be turned off so the program knows there are 2 balls on the play field.
But with 3 balls inserted this switch is constantly turned on which confused the game sequence.

 I can not believe I wasted so much time not seeing the obvious.
But I am a little confused, although happy now, as I thought most Williams machines were designed for 3 balls.